A thermodynamic process of liquefying natural gas to produce liquefied natural gas (LNG) has been developed since the 1970s in order to satisfy various challenges including demands for higher efficiency and larger capacity. In order to satisfy these demands, that is, to increase efficiency and capacity of a liquefaction process, various attempts to liquefy natural gas using different refrigerants or different cycles have been continuously conducted up to now. However, the number of liquefaction processes that are practically used is very small.
One of the liquefaction processes that are being operated and have been most widely used is a ‘propane pre-cooled mixed refrigerant process (or a C3/MR process)’. In the C3/MR process, supply gas is pre-cooled up to approximately 238 K by a multi-stage of propane (C3) Joule-Thomson (JT) cycle. The pre-cooled supply gas is liquefied and sub-cooled up to 123 K by heat exchange with a mixed refrigerant (MR) in a heat exchanger. Since the C3/MR process uses a refrigeration cycle using a single refrigerant and a refrigeration cycle using a mixed refrigerant, the liquefaction process is complex and it is difficult to operate the liquefaction system.
Another one of the successful liquefaction processes that are being operated is based on a cascade process by ‘Conoco Phillips’. The liquefaction process of ‘Conoco Phillips’ consists of three Joule-Thomson cycles using methane C1, ethylene C2, and propane C3, which are pure-component refrigerants. Since the liquefaction process does not use the mixed refrigerant, there are advantages that an operation of the liquefaction process is safe, is simple, and is reliable. However, since each of the three cycles separately requires a compressor, a heat exchanger, and the like, there is a disadvantage that a size of the liquefaction system has no choice but to be increased.
Another one of the liquefaction processes that are being operated is a ‘single mixed refrigerant process (or SMR process)’. In the SMR process, the supply gas is liquefied by heat exchange with the mixed refrigerant in a heat exchanging region. To this end, the SMR process uses a single closed loop refrigeration cycle using the mixed refrigerant. In the above-mentioned refrigeration cycle, after the mixed refrigerant is compressed and pre-cooled, the mixed refrigerant is condensed by the heat exchange in the heat exchanging region and is then expanded. The expanded refrigerant is again introduced into the heat exchanging region so as to condense the pre-cooled mixed refrigerant and liquefy the supply gas. The above-mentioned SMR process has an advantage that a system is compact due to a simple structure, but has a disadvantage in that efficiency of the liquefaction process is bad.